What I've found in the photography world - is that the clients you start out with, doing them a favour, building your portfolio, expanding your client list etc - think is that if you give them work for free initially, then they will always want your work for free. And you become the 'guy that does stuff for free'.
As others say, practice with competitions, build you own website to show off 'made up logos', etc rather than setting the value for your work at zero.